1. Field of the Invention
This invention relates generally to a vacuum pump having a system for controlling the introduction of liquid lubricant into the pumping chamber of the vacuum pump, and more specifically, this invention relates to a control system for precluding the insertion of lubricant into the pumping chamber of a vacuum pump in response to the pressure at the exhaust port of the pump falling below a predetermined level indicative of improper pumping action.
2. Description of the Prior Art
Although the vacuum pumps toward which this invention is directed may be occasionally used as pressure pumps or compressors, their primary usefulness is in the evacuation of vessels, chambers, cavities, etc., in which a very low pressure (high vacuum) is required. In such vacuum pumps, a liquid lubricant, such as oil, is inserted into the pumping chamber. One type of such liquid lubricant insertion arrangement employs a bath of the lubricant in which the stator assembly of the pump is partially immersed, with a lubricant introducing conduit running from the bath to the pumping chamber. Another type of lubricant inserting arrangement employs an oil pump, driven by the same motor shaft that drives the rotor of the vacuum pump, to insert oil into the pumping chamber.
During operation of the pump, the small amount of lubricant inserted into the pumping chamber is exhausted with the gases being expelled through the exhaust port, as a result of the pumping action of the rotor in the pumping chamber, which results in a pressure being developed at the exhaust port. If, for some reason, the pumping action in the pumping chamber is deficient, the pressure at the exhaust port will decrease. Therefore, as a result of the decreased efficiency of the pumping action, the oil being introduced into the pumping chamber will tend to accumulate, since it is not being properly expelled. Similarly, at least in the case where an oil bath is employed, merely stopping operation of the pump will result in an accumulation of lubricant in the pumping chamber. Although various disconnect arrangements are frequently employed between the vacuum pump and the chamber being evacuated, these disconnect devices frequently do not suffice to prevent the accumulated lubricant from being sucked back into the cavity being evacuated, thus contaminating the cavity. Therefore, there exists a real need for a control arrangement that precludes the accumulation of lubricant in the pumping chamber.
As a result of being driven by the motor shaft that drives the rotor of the vacuum pump, an oil insertion pump has a built-in control of lubricant insertion upon deactivation of the motor. However, there may still be some accumulation of oil in the pumping chamber during the period when the rotation of the motor shaft is slowing down, since there will still be some pumping of lubricant into the pumping chamber, but the pumping action may not be sufficiently vigorous to expel all of the lubricant. Further, in other types of pump failure when the motor shaft continues to rotate, at least for a time, after a decrease in the pumping action, oil will accumulate in the pumping chamber of a vacuum pump having an oil insertion pump, with the attendant contamination of the cavity being evacuated.